Modulation circuit organization



Patented Jan. 6, 1942 UNITED vSTA-rss; PATENT o-l-Flca Lothar Brck, Berlin,

', asslsnor to Tele- Germany funken Gesellschaft fr. Drahtlose Telegraphie, Zehlendorf, Osteweg, Germany, a corporatlonof Germany Application May 22, 1940, Serll No. 336,546

In Germany .March 14,

2 claims. (ci. 1791115) ,frequency signals, and the line R of Fig. 2 shows The present invention is concerned with a modulation circuit arrangement forradio-frequency transmitters'. Its outstanding feature that in parallel relation to the oscillation circuit' of a radio-frequency' ampller tube, especially a pentode, or else in parallel relation to a circuit being'coupled with the said oscillation cir# how the effective resistance of the diode circuit varies with the total instantaneous bias on the diode.

R should be .chosen to make the self bias, in absence of modulation, one half the cut-off value.

cuit, there is aA series connection vcomprising a rectifier with linear ycharacteristic (especially a.

diode) and an ohmic resistance of such' size that the family of characteristicsof the rectier is 'operated as symmetrically as possible, and that the modulation potential is impressed upon'the ohmic resistance or upon a transformer connect- ,ed in series with it.

In describing my invention in detail reference will be made to the attached drawing wherein:

Figures 1, 3 and 4 each show a different embodiment of my new and improved modulation means; and f Figure. 2 shows the characteristics of the rectifier as used in my modulation scheme.

The circuit organization as outlined is shown in Fig. l. The radio-frequency or signal energy coming in from the transmtiter is amplied in al tube I which has a high internal resistance.`

The tube I0 may as shown be a pentode. The wave energy raised to a high amplitude is fed from the anode of tube I0 to the oscillation circuit I2 and also to the rectifier tube Il. The

` The audio-frequency potential 1s then superposed upon the direct-current voltage U, and the operation for the family of characteristics is to be as symmetric as possible, with a view to obf taining as high a modulation percentage as feasible, in the presence of reduced distortion. A high resistance R' providing a characteristic which correspondsto the straight (dash) line would, therefore, be wrong. The inclination of the rectification characteristics is a function of the damping in thc radio-frequency circuit. Its damping, as far as possible, should be due only to the rectifier load and thus its effective (ohmic) resistance be R/Z. It is for this reason that the rectiiier tube I4 operates inside the straight portion of its characteristic obtained by plotting the direct current component, I=, as a function'of the high frequency voltage UHF. The rectified radio 'frequency produces across the resistance R the direct-current potential U.

Fig. 2 shows the family of rectificationcharacteristics where I=, the direct current coin'- ponent, is a function of U, the direct current voltage (at constant UHF) with operating point A. In'Fig. 2 each-of the lines sloping up to the right represents, f or a particular value of radiofrequency voltage across I2, the relationbetween rectified current and the, direct-current component of voltage between anode and cathode of the rectifier. As this bias is made more negative (abscissa) the rectied current (ordinate) is reduced. The line gets down to zero when this bias equals the peak value of the radiofrequency across I2.

Where audio frequency is superposed on the drop thru resistance R (which drop produces the bias U) then the rectified current is varied. But this is the same, in' its eiIect on tank l2,

input tube l0 is a pentode. The circuit 2 could also be replaced by a band-pass filter or a cir cuit in loose coupling relationship with the transmitter. Modulation is produced by varying the effective resistance R/2 of the circuit at the rate and-rhythm of the modulating potential applied across R. As a consequence the radio-frequency resistance in the plate circuit of the input tube I0 is variable and the radio-frequency potential arising across the same fiuctuates at the rate and rhythm of the modulation potential.

The audio-frequency potential'may be'introduced alsc in the manner shown in Fig; 3 by means of a transformer I6. The sum total of' the ohmic resistance of the transformer winding S and RI must be `equal to the resistance R in Fig. 1. Somewhat more suitable as regards the frequency 'characteristic' than circuit scheme Fig. 3 is the circuit organization of Fig. 4 where Cl is a modulation frequency shuntof Rl, while C2 is a radio-frequency by-pass for the transformer secondary S.

What is claimed is:

1. In a modulation system, an electron discharge tube of the screen grid type having input electrodes excited by radio frequency energy and having output electrodes, a radio frequency cir.- cuit parallel tuned to the frequency of said Wave energy coupled to said4 output electrodes, a rectiiler and aresistance in series coupled in shunt to said radio frequency circuit, a source of modulating potentials, and means coupling said source u th iz f R n d b th di u' of moduiatingpontials in shunt to-said resist- S e880 Welevae y B8110- v- 2. In a modulation system, an electron discharge amplifier tube having input electrodes excited by wave energy of carrier Wave frequency and having output electrodes from which modulated wave energy may be derived, a circuit parallel tuned to the frequency of the said waveenergy connected between said output electrodes, a. diode rectifier and a resistance shunted by a condenser in series across said parallel tuned circuit, the value of said resistance being such that a direct current diode biasing potential is developed therein which is approximately half that necessary to block said diode in the presence of the peak rad1o frequency voltage and in the absence of modulation, and means for feeding modulating potentials to said resistance.

LOTHAR, BRCK. 

